A brake system in accordance with this invention includes:    a) a brake pedal for actuating a brake master cylinder having a housing and two pistons which are arranged one behind the other and delimit pressure chambers arranged in the housing, to which pressure chambers two brake circuits are connected, an actuating force (pedal force) being exerted on the pistons upon actuation of the brake system by the vehicle driver and the pistons being positioned in a starting position by return springs when the brake pedal is not actuated,    b) a pressure medium reservoir which is under atmospheric pressure and has chambers associated with the pressure chambers,    c) a travel detection device which detects the actuation travel of the brake pedal or of a piston connected to the brake pedal,    d) a travel simulator having a simulator release valve, which travel simulator communicates the accustomed brake pedal feel (haptic) to the vehicle driver in the brake-by-wire operating mode, being connected hydraulically via the simulator release valve to one of the pressure chambers and this connection being disconnected in the fallback operating mode,    e) an electrically controllable pressure source which delivers a brake system pressure,    f) isolation valves for isolating the pressure chambers from the brake circuits,    g) pumps which are driven by means of an electric motor and are associated with the brake circuits, together with low-pressure hydraulic accumulators,    h) an inlet valve and an outlet valve for each wheel brake for setting wheel-individual brake pressures which are derived from modulator admission pressures associated with the brake circuits, the inlet valves transmitting the modulator admission pressure to the wheel brakes in the unactivated state and limiting or preventing a build-up of wheel brake pressure in the activated state and the outlet valves preventing an outflow of pressure medium from the wheel brakes into a low-pressure accumulator in the unactivated state and permitting and controlling the outflow in the activated state, the inlet valves being closed, so that a reduction of wheel brake pressure takes place,    i) electrically operable, currentlessly open valves connected to the outlet ports of the pumps, and    j) at least one electronic control and regulation unit.
A brake system of the above-referenced general type is known, for example, from DE 10 2009 031 392 A1. In the known brake system, two pressure chambers of a brake master cylinder are connected to two respective pressure chambers of a dual-circuit electrically controllable electrohydraulic pressure source.
The two pressure chambers of the electrohydraulic pressure source are connected via an ABS system to four wheel cylinders of the wheel brakes. The wheel cylinders are actuated by brake fluid pressure which, in a malfunction situation in which the electrically controllable pressure source is inoperative, is generated by the brake master cylinder. The known brake system includes two separate, independent fluid pressure lines: one line extends from one of the pressure chambers of the brake master cylinder to the wheel cylinders via one of the pressure chambers of the controllable pressure source, and the other line extends from the other pressure chamber of the brake master cylinder to the second pair of wheel cylinders via the second pressure chamber of the controllable pressure source. A minimum required braking force is therefore ensured even if a malfunction occurs in one of the two fluid pressure lines.
It is disadvantageous that, in an operating mode without actuation of the electrically controllable pressure source, not just a minimum braking force but practically no braking force is actually available if leakage occurs in one of the two brake circuits. As is apparent from each of FIGS. 1 to 6 of the aforementioned document, the two pressure chambers of the electrically controllable pressure source are separated from one another by a floating piston which is freely displaceable in a cylinder. The pressure medium volume displaced from the master cylinder into the cylinder in the intact brake circuit therefore causes merely a displacement of the floating piston but not a build-up of wheel brake pressure.
Starting from this state of the art, it is the object of the present invention to develop a brake system which in this operating mode is able to build up pressure in the remaining brake circuit in the event of failure of one brake circuit.
This object is achieved according to the present invention by a brake system as described herein.